Enhanced light emission from a Si photonics beam steering device consisting of asymmetric photonic crystal waveguide

Abstract

We have studied a Si photonics non-mechanical beam steering device for LiDARs. We exploit a doubly periodic Si photonic crystal waveguide (PCW) with a collimator lens, which emits a single-peaked optical beam. Thanks to the slow light effect in the PCW, wide range beam steering can be obtained in the longitudinal direction with maintaining a small beam divergence by a small change of the wavelength and/or index of the PCW. However, due to the symmetric crosssection of the PCW, the emission occurs in both upward and downward directions, which causes a 3-dB loss in the transmission of the optical beam. The downward beam is partly reflected by the substrate, and the reflected beam interferes with the upward beam and modifies the far field pattern, which further increases the loss at particular beam angles. In LiDARs, this loss is repeated at the reception of returned light, resulting in a severe loss penalty. In this study, we investigated the unidirectional upward emission in some PCW structures with vertical asymmetries. We found theoretically that a shallow etched grating on top of the Si layer, which overlaps with the PCW holes significantly increases the upward emission. We fabricated such a device using Si photonics CMOS process and observed 2-8 times stronger upward emission as compared with that of the symmetric PCW. Furthermore, we integrated 32 PCWs in parallel configuration and selected one working PCW so that its relative position against a collimator lens is switched and the beam is steered in the lateral direction. We observed over 400×32 resolution points.